The present invention relates to a process for generating, from refuse or refuse in combination with coal, a fuel gas suited for public utility purposes.
For the purposes of the present invention, the term refuse is used to describe refuse containing energy, for example domestic refuse and other refuse, such as industrial refuse, including for example also waste wood and other kinds of biomass. The term refuse containing energy as used herein is meant to describe refuse containing carbon.
There have been known substantially three different methods for the thermal utilization of refuse:
1. Refuse incineration PA1 2. Low-temperature carbonization of refuse PA1 The refuse is burnt in a furnace, and the heat contained in the flue gas is transferred to a steam boiler. PA1 The energy contained in the steam is used to produce electric current and remote heat. PA1 This utilization requires a complex electric remote distribution system and remote heat network. PA1 The efficiency of the electric power generation process is low, due to the unfavorable handling properties of the refuse. PA1 As regards the separation of re-usable components, this is substantially limited to lumpy ferrous metals which can be separated and recycled. PA1 All the other metals get into the incineration process. PA1 The reactive substances, such as sulfur and heavy metals, assume their mostly toxic oxide state--this process being aided by the required high rate of excess air--and have to be removed from great volumes--because of the excess air--of flue gas by a complex cleaning process. PA1 Sulfur dioxide develops which is converted to gypsum by the addition of lime, and the gypsum then has to be disposed of by costly processes. PA1 The denitrogenation process is rather difficult due to the fact that the catalytic poisons still contained in the flue gas heavily impair the service life of the catalysts. PA1 The present state of the art is not in a position to propose denitrogenation plants for refuse incineration processes. PA1 The nitrogen oxides emitted at great height produce photo-oxidants (ozone) under the effect of the sun, thus contributing to the problem of dying forests. PA1 Due to the high oxygen excess in the presence of numerous other chemical substances, there is also the risk that extremely toxic organic contaminants (such as dioxine) may develop. PA1 a) With subsequent combustion of the low-temperature carbonization products PA1 b) With a subsequent refining step and combustion of the low-temperature gas PA1 producing from the refuse a low-temperature gas and a low-temperature carbonization residue in an externally heated low-temperature gas generator, in the absence of air and/or oxygen (allothermal process) and without any addition of gasification media, PA1 reducing the low-temperature carbonization residue, either without the addition of coal or with the addition of a desired quantity of coal, in a mill to a fine-grained product, hereafter called dust, PA1 generating from the low-temperature gas and the dust a crude gas and a gasification residue, in an externally heated crude gas generator, in the absence of air and/or oxygen (allothermal process) using water as a gasification medium, PA1 producing a pure gas from the crude gas by purification, and PA1 producing from the gasification residue a flue gas (thermal gas) in a gasification-residue combustion chamber. PA1 that a fuel gas suited for public utility purposes can be produced from the refuse, and that the process is not restricted --as are all the other refuse utilization processes described before--to the supply of electric current and remote heat; PA1 that due to the fact that the energy is made available in gaseous form, substantial costs can be saved, Which would otherwise be caused by electric current and remote-heat distribution systems, and the generation of electric current and heat can be decentralized; PA1 that the possibility to decentralize the generation of electric current helps save stand-by units; PA1 that the possibility to make use of decentralized power stations reduces the need for large power stations and allows gas generators to be erected at the sites of abandoned coal-fired power stations; PA1 that it is now possible to offer a low-cost and competitive coal gasification process in combination with the--anyway indispensable--disposal of refuse; PA1 that--for covering peak loads in public gas networks--coal of any grade can be used, in any mixing ratio With the refuse to be disposed of, and that the plants are no longer restricted either to refuse or to coal, as is the case with all other thermal processes for the utilization of refuse or coal; PA1 that approximately 95% of the energy contained in the refuse can be put to useful purposes through the carbonization process and that--in contrast to the process described under 2 b above--no substantial part of the energy content remains in the residue which cannot be utilized energetically; PA1 that the gas purification process can be carried out on the smallest possible gas volume and not on the large--as in the case of the processes 1 and 2 a--or enlarged gas volume--as in the case of process 2 b; PA1 that the gas purification process is relatively unproblematic due to the fact that approximately 5% only of the reactive substances are subjected to combustion so that oxidation will occur only to this extent--as compared to the 100% in the case of processes 1 and 2 a; PA1 that the flue gas volume is extremely small, equaling only 3% of that produced by processes 1 and 2 a; PA1 that for removing the sulfur content (from the crude gas), a simple liquid purification process is required only, as compared with the expensive and complex flue-gas desulfurization system comprising the use of lime and the disposal of gypsum, as required for processes 1 and 2 a; PA1 that when burning the flue gas at decentralized consumer points, this can be done without any catalysts due to the low-pollutant hydrogen combustion, and the internal-combustion engines of the power stations can be operated on a lean mixture, without any need for catalysts, as in the case of the processes 1 and 2 a; PA1 that due to the reduction of the combustion share to approximately 5% and the possibility to make use of the fluid-bed combustion process, the emissions of nitrogen oxide can be expected to be small and no substantial emissions of nitrogen oxides have to be expected due to high combustion temperatures, and this even at great geodetical height (high chimneys--development of photo-oxidants--dying forests)--as in the case of process 1 and, in particular, process 2 a.
a) with subsequent combustion of the low-temperature carbonization products PA2 b) with a subsequent refining step and combustion of the low-temperature gas. PA2 The carbonization residues are ground in a mill, burnt together with the low-temperature gas in a high-temperature slag-tap furnace, and the heat contained in the flue gas is transmitted to a steam boiler. PA2 The energy contained in the steam is used to produce electric current and remote heat. PA2 This utilization requires a complex electric remote distribution system and remote heat network. PA2 The efficiency of the generation of electric current should be more favorable as compared with the process described under 1) above, due to the better handling properties of the low-temperature carbonization products, as compared with refuse. PA2 During the combustion process, certain components of carbonization residues change over to their mostly toxic oxide form and have to be removed from the flue gas--which in this case, too, arises in quite considerable volumes, in spite of the low rate of excess air, by a complex cleaning process. PA2 Sulfur dioxide develops which is converted to gypsum by the addition of lime, and the gypsum then has to be disposed of by complex processes. PA2 The high-temperature combustion gives rise to large quantities of nitrogen oxides. PA2 As regards the removal of nitrogen in high-temperature slag-tap furnaces, no satisfactory solution has been found to this day. PA2 The nitrogen oxides emitted at great height produce photo-oxides (ozone) under the effect of the sun, thus contributing to the problem of dying forests. PA2 The low-temperature gas is refined by partial combustion (autothermal process) in the presence of air, and the resulting crude gas is purified. During the refining process, large-molecular carbon compounds are reduced to small-molecular carbon compounds. PA2 The pure gas is a poor gas, due to the nitrogen absorbed in the refining stage, and is not suited for public utility purposes. PA2 The pure gas can be converted to electric current and remote heat only at the very place. PA2 A complex electric distribution network and a complex remote heat network are required. PA2 The low-temperature carbonization residue, which still contains a substantial part of the energy content of the refuse introduced into the process, cannot be further utilized thermally, due to the pollutant emissions that have to be expected. PA2 The energetic efficiency of the process, therefore, is only low. PA2 In the refining stage, the gas volume is increased, and as a result the purification plants also have to handle larger volumes. PA2 However, due to the (below-stoichiometric) air supply, undesirable oxidations can hardly develop. PA2 Sulfur is present in the crude gas practically only in the form of hydrogen sulfide and can be washed out simply by liquid purification. PA2 Elementary sulfur of a high purity degree can be recovered. PA2 Thanks to the existing possibilities of catalytic reduction, nitrogen oxides are produced in only very small quantities by the combustion of the pure gas.
Hereafter, the typical forms of implementation of the known methods will be described, together with their disadvantageous results and effects: